The fabrication of high performance nanoscale materials useful for ultra sensitive molecular imaging, early stage diagnosis and targeted therapy is of great interest.1-4 Smart nanomaterials that can integrate different properties and functionalities are especially desirable.4,5 For example, nanomaterials that combine the complementary features of magnetic resonance imaging (MRI) and optical imaging could lead to significant technological advantages for molecular imaging and diagnostics, since MRI provides exquisite spatial-resolution and anatomical contrast, while optical imaging is highly sensitive, less costly, and easier to implement.4,6-9 Pioneering studies have demonstrated clear synergies between these two imaging modalities, however, the magnetic and optical properties of the underlying nanomaterials are still far from optimal.10-12 In addition, none of the reported magnetic and optical nanomaterials have shown advantageous imaging capabilities at the single molecule level.